This invention relates to improvements in activated-sludge sewage treatment systems. This invention more specifically relates to activated-sludge sewage treatment systems in which all or part of the aeration basin is a long, narrow chamber designed to promote plug flow of the mixed raw sewage and recycle sludge from the inlet end of the chamber to the outlet end. Such plug flow permits the introduction of more air per unit volume near the inlet end in order to give a large reduction of biological oxygen demand (B.O.D.) per unit volume of aeration chamber, and less air per unit volume near the outlet end in order to save the cost of pumping air which would not be utilized because there is not enough food left in the mixed liquor to consume a large amount of available oxygen. This invention is particularly applicable to activated-sludge sewage treatment systems designed to produce an effluent with a B.O.D. content substantially less than ten parts per million (10 ppm).
Sewage treatment processes remove undesirable or offensive waste from water. Primary sewage treatment removes solids from the water by using screens, grit chambers, skimming tanks, and settling basins. Secondary sewage treatment generally is preceded by primary treatment. It is a process whereby a biological treatment system rapidly breaks down organic material. If the effluent of the secondary sewage treatment is not sufficiently clean to meet the mandated standard, secondary treatment may be followed by tertiary treatment.
The activated-sludge sewage treatment system is a commonly used form of secondary treatment. It uses biologically active growths as a means to process raw sewage into relatively clean water. This microbiological culture is mixed with raw sewage (or the effluent of a primary clarifier) in a basin or chamber. Aeration means provide sufficient air to promote consumption of the colloidal and soluble organic matter (i.e., biologically degradable waste) in the sewage by the culture. When the microbes feed upon the organic matter in the sewage, they generate an additional mass of microorganisms (referred to as "activated sludge"), along with carbon dioxide, water, nitrogen compounds, and traces of other compounds. When substantially all of the colloidal and soluble matter has been converted to insoluble microbes and innocuous by-products, the mixture is directed to a clarifier or secondary settling tank, which separates the relatively clean water, or finally treated effluent, from the microbes, and allows the clean water to be decanted. The finally treated effluent is then released into a river or an intermittent stream. A substantial portion of the activated sludge is recycled to the aeration basin, while some of the sludge is continuously withdrawn to avoid excessive accumulation of the recycle sludge.
For this system to produce a good quality of treated effluent, the decantation step must remove more than 99% of the solids from the feed mixture. Occasionally, the microbiological growth produces a filamentous mycelium which settles very slowly, if at all. Filamentous mycelia in the effluent of the aeration chamber make it impossible to get a good quality of treated sewage from the decanter (clarifier). This filamentous growth is caused by various factors, but most often by too much or too little air. Penury dictates that if there is an imbalance of oxygen demand and oxygen supply, the error will almost always be a short oxygen supply. Once a filamentous growth starts, it is difficult to suppress. In a large aeration basin with an adequate air supply, it is possible to have localized areas of oxygen starvation which invite filamentous growth. The designing engineer must avoid this pitfall.
Traditionally the aeration basin has been a narrow, long chamber designed to promote plug hydraulic flow. Typical dimensions are from 20 feet by 200 feet to 40 feet by 1,000 feet, with water depths of 12 feet to 18 feet. For economy of land use and of construction costs, the longer chambers are usually built in three parallel sections with a common wall between sections. The plug hydraulic flow of the mixed liquor through the aeration basin insures the maximum reduction of pollutants in the clarified effluent, while maintaining a high rate of oxygen usage throughout most of the chamber volume. In fact, one of the problems with the plug-flow aeration basin is the tendency to grow filamentous mycelia in spots of localized oxygen starvation.
In the past thirty years, the complete-mix aeration basin has been developed to compete with the plug-flow aeration basin. In the complete-mix aeration basin, all mycelial growth occurs in a medium with substantially the same concentration of B.O.D. as that of the clarified effluent. It is thus easy to avoid spots of localized oxygen starvation and the attendant filamentous mycelial growth.
While longitudinal recirculation is discouraged in a plug-flow aeration basin, transverse mixing is promoted, to transfer oxygen from the point of introduction to the body of the mixed liquor, and to prevent settling of the activated sludge. The soluble and colloidal pollutants are quite evenly distributed throughout the transverse section, and it is necessary to bring oxygen and microbes to all parts of the chamber in order to consume the pollutants. Currently in the United States, this is done by "spiral-roll aeration" as shown in FIG. 1. In this system, a series of air spargers or diffusers is located near the bottom of one long wall of the chamber. The rising air bubbles carry a stream of water up this wall. Gravity carries this water across the top of the pool, down the opposite wall, and across the floor of the basin back to the air spargers. The turbulence of this process holds the activated sludge in suspension. Ideally, a droplet of water follows a spiral course from the inlet of the aeration chamber to the outlet, sweeping the activated sludge with it and repeatedly absorbing oxygen as it passes the air spargers.
Past practice in the United States has been to supply air to an activated-sludge aeration basin at a fairly constant rate over the 24-hour period, even though the hourly rate of inflow of raw sewage tends to vary widely during this time, as shown in FIG. 2. Since the sewage usually has a higher concentration of B.O.D. at the time of the higher flow rate, the ratio of the highest hourly input of B.O.D. to the lowest hourly input of B.O.D. during the diurnal cycle may be six to one, or even eight to one, although the ratio of the highest hourly flow rate to the lowest hourly flow rate is perhaps only four to one.
Although, with the constant rate of air flow to the aeration basin, the activated sludge adsorbs some of the excess oxygen during periods of low B.O.D. inflow and releases it during periods of high B.O.D. inflow; this system of control of air flow results in pumping more air than is needed to promote the biological growth, and/or gives a treated-sewage effluent with a B.O.D. content which varies widely over the diurnal cycle. Hopefully, the daily average B.O.D. content will meet the 20 ppm standard of the United States Environmental Protection Agency. In recent years, some United States and foreign municipalities have begun using computer control to adjust the rate of air flow in response to the immediate rate of B.O.D. input to the aeration basin, in order to reduce the cost of pumping air and/or to produce a treated-sewage effluent of consistently good quality. Unfortunately, when the air flow is reduced to match the low B.O.D. input in the early hours of the morning, there is insufficient transverse movement of the mixed liquor to hold the activated sludge in suspension, and sludge may accumulate on the floor of the basin across from the air diffusers, as shown in FIG. 1. This effect is most pronounced near the exit end of a plug-flow aeration basin designed to produce an effluent with less than 10 ppm of B.O.D., where the air flow required for oxygen supply during the hours of maximum rate of B.O.D. input is barely sufficient to maintain the activated sludge in suspension. The settled sludge contributes nothing to the pollution-abatement process, and by depleting the local oxygen supply, may initiate the growth of filamentous mycelia.
Near the inlet end of a plug-flow aeration basin, where there is much food for biological growth, some of the mycelia tend to grow in clumps. These clumps are difficult to keep suspended in the mixed liquor, and present a reduced amount of surface for absorbing oxygen and nutrients.